A precision approach path indicator (PAPI) is equipment installed at an airfield to guide pilots during their approach (e.g., descent) to the runway for a safe landing. The PAPI guides the pilot by providing the pilot with the correct glide path (e.g., angle) to take during the approach to the runway.
A PAPI generally includes four light emitting units placed at the side of the runway. Each PAPI unit emits a circular beam of light, the lower half of which is red and the upper half of which is white. The inclination angle of each PAPI unit is set (e.g., fixed) such that the pilot will see the two lights closest to the runway as red and the other two lights as white when the pilot is on the correct glide path during the approach. If the pilot sees three of the lights as one color and the other light as the other color, or if the pilot sees all four of the lights as the same color, that indicates the pilot is not on the correct glide path, and needs to alter his glide path accordingly.
In previous approaches, the inclination angle of a PAPI unit may be manually (e.g., mechanically) measured and set using, for example, an inclinometer or clinometer. However, the inclination angle of the PAPI unit may not be aligned with the inclination angle of the beam of light emitted from the PAPI unit (e.g., there may be a misalignment between the mechanical axis of the PAPI unit and the optical axis of the PAPI unit). Accordingly, manually measuring and setting the inclination angle of the PAPI unit using an inclinometer or clinometer may result in the PAPI unit providing an incorrect (e.g., inaccurate) glide path (e.g., a glide path that differs from the correct glide path by at least five minutes of arc). That is, a PAPI unit whose inclination angle is manually measured and set (e.g., using an inclinometer or clinometer) may not provide the correct glide path (e.g., a glide path that differs from the correct glide path by less than five minutes of arc).